The present invention relates to the recovery of silver from photographic materials. The invention further relates to a new process for recovering metallic silver from scrap materials and particularly from scrap materials in which substantial portions of the silver is combined as halide salts, such as in photographic emulsion scrap materials.
Used and green non-used photographic emulsion materials from the photographic industries take a number of forms, including batches of photographic emulsions which are separated from or which have never been combined with a film or paper substrate, unexposed emulsions on a film or paper substrate and exposed emulsions on a film or paper substrate. The most valuable constituent of these scrap materials is the silver, however, the film substrate material can be reclaimed under appropriate conditions.
Various methods have been used to recover silver from photographic materials such as burning the scrap emulsion in a furnace. Other processes for reclaiming silver from photographic substrates have been used, and some of these processes have involved complicated and expensive equipment such as centrifuges for the purposes of separating silver from liquids in which the silver is precipitated, after being removed from the photographic substrate material such as plastic film, paper or the like. Centrifuges are expensive and require careful attention as well as proper maintenance.
Other methods utilize chemical solution treatments such as the use of sodium cyanide or sodium thiosulfate but these chemical compounds present severe disposal problems and are toxic. The common methods of burning the materials or chemically treating photographic materials in a stripping fashion all involve a number of serious problems and disadvantages. For instance, the burning of emulsion materials produces serious air pollution, since the silver is present in the emulsion in a proportion of only about 1 to about 7 percent by weight and most of the remainder, other than moisture content consists of organic materials such as animal fat, protein, gelatin, natural fine bone and wood pulp. Furthermore, as much as 10 to 20 percent of the silver is lost in the smoke emitted upon burning operations and therefore cannot be subsequently recovered in the smelter. Because the burning process has been regarded as an important method by which the silver can be recovered, tremendous investments have been made in facilities for burning huge quantities of photographic emulsion scrap while taking special precautions to try to minimize the resulting air pollution and loss of silver as dust in the flue gases.
On the other hand, in the smelting of chemically produced precipitates, gathered from the stripping and washing of silver from photographic substrates, various fluxes have been used such as sodium carbonate or sodium ash and borax. With such prior art fluxes, high alkalinity destroys the crucibles and in this fashion the prior art methods for smelting of the reclai silver materials have been relatively costly and presented environmental problems. Chemical solution methods according to the prior art provide for the washing of x-ray films and other photographic materials on similar substrates with silver halides contained in the emulsions, in dilute solutions of sodium hydrochloride until the silver bearing emulsion is removed from the substrates so that the silver is contained in the solution. The solution may then be placed in a settling tank with the sodium hydroxide to precipitate the silver. However, flocculating agents must also be added to flocculate the particles of silver such that the silver in the settling tank forms into a sludge in the lower strata thereof. The use of bleach as a stripping agent creates a processing burden in the form of uncontrollable foam and the foam interferes with the recovery of silver. The build up of animal protein in solution stripping of photographic emulsions is attacked by the bleach and creates even more foam thus the prior art has utilized defoaming agents, however, such agents interfere with the rapid processing required for economical removal of silver.
Additional prior art methods for recovering silver from photographic materials have included treating the scrap materials with hot caustic material solutions and with solvents. Generally the cut photographic film is sized to small pieces of film and with the caustic aqueous solution forms a slurry. The slurry is then fed to a classification and/or separating system.
It is also known that silver halide emulsions can be readily stripped from various substrates by simply subjecting the film to hot water. The principal draw back to this and other prior art silver recovery techniques is that they are not efficient or frequently use toxic materials. Also, modern emulsions include other polymeric layers which complicate the recovery of the silver. Moreover, merely subjecting small pieces of scrap film or photographic paper to hot caustic or hot water or a solvent, and stirring the mixture in the manner described by the prior art can cause the pieces to stick together when they contact one another so that the high degree of agglomeration occurs. This provides an extremely difficult problem in removing all the silver emulsion. Another problem in prior art processes for recovery of silver from photographic scrap has been that different processes are required for different types of scrap. For instance, the silver content of exposed scrap film is largely in the form of metallic silver, while the silver content of unexposed scrap film is in the form of silver combined as silver halides such as silver bromides. All of these different forms of scrap require different processing steps.
It is an object of the present invention to provide a process which is capable of recovering silver from any of the above forms of photographic scrap, and furthermore is capable of recovering silver from batches of photographic scrap which contain any or all of the above forms of photographic scrap in combination.
It is another object of the present invention to provide an improved process for the recovery of silver from photographic emulsion scrap in which the steps of separating the silver content of the photographic emulsion is achieved in a non-toxic environment.
It is yet another object of the present invention to provide a stripping formulation for removing photographic silver emulsions from paper or film substrates which avoids precipitation of the silver material during stripping and substrate separation.